Wavelength tunable laser

ABSTRACT

A wavelength tunable laser comprising a laser diode and a closed external cavity formed by one or more optical resonators either horizontally or vertically coupled to adjacent waveguides. The optical resonator primarily functions as a wavelength selector and may be in the form of disk, ring or other closed cavity geometries. The emission from one end of the laser diode is coupled into the first waveguide using optical lens or butt-joint method and transferred to the second waveguide through evanescent coupling between the waveguides and optical resonator. A mirror system or high reflection coating at the end of the second waveguide reflects the light backwards into the system resulting in a closed optical cavity. Lasing can be achieved when the optical gain overcomes the optical loss in this closed cavity for a certain resonance wavelength which is tunable by changing the resonance condition of the optical resonator through reversed biased voltage or current injection. Multiple optical resonators may be used to reduce the lasing threshold and provide higher power output. With monolithic integration, more optical devices can be integrated with the tunable laser into the same substrate to produce optical devices that are capable of more complex functions, such as tunable transmitters or waveguide buses.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending application Ser. No.10/077,522 filed Feb. 15, 2002, which application is fully incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of optical communications andmore particularly, to a light source with tunable wavelength for use inoptical communications systems.

BACKGROUND OF THE INVENTION

The rapid growth of Internet data traffic has driven current fiber opticnetworks to a new stage where much broader bandwidth and higher capacityare required. Dense wavelength division multiplexed (DWDM) systems withnarrow channel spacing and low crosstalk have proven to be a promisingsolution. Generally in a DWDM system, each channel is represented by afixed wavelength from a wavelength-fixed laser source and all thedifferent channels are sent into the same optical fiber and transmittedto a receiver end. In order to fully implement a DWDM system withthousands of channels under this wavelength-fixed scheme, serviceproviders in the telecom industry face huge inventory, complexity, andcost problems because of the large number of laser sources andaccessories needed, as well as the need for backup lasers and spareparts. However, the use of tunable laser sources in which the lasingwavelength can be tuned over a certain range, for instance, thewavelength band of erbium doped optical-fiber amplifier (EDFA), coulddramatically simplify a DWDM system, enable highly flexible andeffective utilization of the optical fiber bandwidth, and, thus, cansignificantly reduce cost to service providers.

Several tunable laser technologies have been investigated over the lastdecade including distributed feedback Bragg (DFB) grating lasers,distributed Bragg reflector (DBR) laser, vertical-cavitysurface-emitting lasers (VCSELs), and external cavity lasers (ECLs).Tunable DFB lasers are in general realized by changing the refractiveindex of the internal grating either thermally or electrically by whichthe operating wavelength can be tuned. Although DFB lasers are wellbehaved and very reliable, they have the disadvantages of low outputpower and very limited wavelength tuning range (i.e., a range of about5.0 nm).

DBR lasers have similar structures to DFB lasers but have a gratingsection separated from an active section. By injecting current into thegrating region to change the refractive index, the effective length ofthe laser cavity is changed and therefore the lasing wavelength. DBRlasers have some advantages such as fast tuning speed, relatively largetuning range (about 40 nm), but suffer drawbacks of wavelengthinstability, broad linewidth, and large device size.

VCSELs have a gain layer sandwiched by two DBR mirrors. The light isemitted from the top surface of the mirror instead of the edge as in theconventional edge-emitting lasers. This gives VCSELs the biggestadvantage in that the laser output can be coupled to a fiber very easilyand cost-effectively. The wavelength tuning of VCSELs is realized byinjecting current to a micro-electromechanical-systems (MEMS) cantileverintegrated with the top DBR mirror thereby changing the cavitythickness. The use of MEMS tends to limit the tuning speed of the devicewithin the microsecond range. However, the main disadvantage of VCSELsis that they tend to have low output power (i.e., on the order of abouthundreds of microwatts or lower). Another disadvantage of traditionalVCSELs is their operational wavelengths are limited to short wavelengthsof about 850 nm to about 1300 nm.

ECLs basically utilize an external reflector such as a diffractinggrating or MEMS mirror to form an external cavity. By mechanicallyadjusting the external cavity length, the lasing wavelength can be tunedover a wide range. ECLs can also provide high output power and narrowlinewidth. However, most of current ECLs are very large, costly,sensitive to environmental changes, and operate with a slow tuning speedon the order of milliseconds. In addition, current ECL designs tend notto be applicable to large-scaled integration.

SUMMARY OF THE INVENTION

The present invention is directed to an improved tunable wavelengthlight source for use in optical communications systems that facilitateshigh speed, broad band wavelength tuning, is mechanically simple,scaleable and reliable, and facilitates monolithic integration ofoptical components. The novel wavelength tunable light source of thepresent invention preferably comprises a semiconductor laser diodeoptically coupled to a tunable wavelength selective external cavity.Preferably, the external cavity comprises a waveguide-coupled opticalresonator that includes an optical resonator, or multiple opticalresonators, either horizontally or vertically coupled to adjacentsemiconductor waveguides. The optical resonator, which is preferablyformed from electro-optic materials, primarily functions as a wavelengthselector and can be in the form of a disk, ring or other closed cavitygeometries. In operation, light signal emissions from one end of thesemiconductor laser may be coupled into a first waveguide using anoptical lens or butt-joint method, and then transferred to a secondwaveguide through evanescent coupling between the waveguides and theoptical resonator when the wavelength of the light signal is at aresonance frequency of the resonator. A mirror system or high reflectioncoating at the end of the second waveguide reflects the light signalback into the system. A closed optical cavity is realized as a result.

Lasing can be achieved in the light source of the present invention whenthe optical gain overcomes the optical loss in the closed cavity for acertain resonance wavelength. The resonance wavelength is preferablytunable by changing the resonance condition of the optical resonatorthrough current or reversed biased voltage. For a given material andstructure, the wavelength tunable range tends to be determinable by thesize of the resonator. The use of multiple optical resonatorsadvantageously tends to reduce the lasing threshold and tends to providehigher power output.

When compared to traditional ECL designs, the wavelength tunable laserof the present invention tends to possess several advantages. Forinstance, the present invention tends to be smaller in size because ofits use of compact waveguide-coupled optical resonators as the externalcavity. The tunable laser of the present invention also tends to havemuch faster tuning speeds because the tuning mechanisms useelectro-optic effects or carrier effects instead of thermal or MEMSeffects. Lastly, the semiconductor laser diode and the opticalresonators forming the tunable laser of the present invention may befabricated on the same substrate and, thus, facilitate monolithicintegration.

In preferred embodiments, the semiconductor laser diode has one endfacet anti-reflection coated from which light emissions are coupled intothe first waveguide of the waveguide-coupled optical resonator using ahigh numeric aperture lens. The two coupling waveguides of thewaveguide-coupled optical resonator are preferably designed, forexample, using a tapered structure to take advantage of mode matchingbetween the waveguide and optical resonators in the interaction regionof the device. A high reflection coating is applied at one end of thesecond waveguide forming a closed optical cavity. As a result, the powertransferred from the first waveguide through the optical resonator(s)can be totally reflected back into the system. When one of the cavitymodes (represented by a particular wavelength or frequency) is atresonance with the optical resonator(s), i.e., the wavelength is one ofthe resonance wavelengths of the optical resonator, and has enoughoptical gain through current injection into the laser diode tocompensate for the optical loss encountered in the closed cavity, lasingfrom the other end facet of the laser diode may be achieved at thisparticular wavelength. The resonance wavelength of the opticalresonators may be adjusted or tuned by applying voltage or injectingcurrent to the resonator. Therefore, the lasing wavelength of thetunable laser of the present invention may be tuned. The tuning range isdetermined by the free spectral range of the optical resonators.

In other preferred embodiments, a mirror with 100% reflectivity placedright after the end facet of the second waveguide reflects thetransferred power back into the system. This mirror could be movable toadjust the distance between the mirror and the waveguide and, thus,change the length of the cavity.

In yet other preferred embodiments, the coupling waveguides may includean active medium. By injecting current into one or more sections of thecoupling waveguides, for example, at the end of the second waveguide,additional optical gain is provided for the light and lower thresholdlasing can be achieved.

A significant advantage of the tunable laser of the present invention,as compared to other ECL designs, is that monolithic integration tendsto be possible. For example, the laser diode is fabricated first on anInP substrate. The optical resonators and coupling waveguides are thenfabricated on the same substrate by using regrowth methods and otherstandard semiconductor fabrication processes. The emission of the laserdiode is coupled into the first coupling waveguide through butt-jointmethodology. In the same way, other optical devices can be integratedonto the same substrate and more complex functions can be realized. Forexample, an electro-absorption modulator may be fabricated right afterthe tunable laser wherein the output from the tunable laser may bemodulated or another waveguide-coupled optical resonator may be coupledto the outputs from different tunable lasers to multiplex these outputs.

Other aspects and features of the present invention will become apparentfrom consideration of the following description taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an edge-emitting Fabry-Perot laser diodewith a typical pin wafer structure with two end facets serving as thetwo reflective mirrors of a laser cavity by cleaving the laser chip.

FIG. 2A is a schematic view of a waveguide-coupled optical resonatorsystem with two waveguides horizontally coupled to a resonator.

FIG. 2B is a cross-sectional view of the waveguide-coupled resonatorsystem shown in FIG. 2A and taken along line 2B-2B.

FIG. 3A is a schematic view of a waveguide-coupled optical resonatorsystem with two waveguides positioned below and vertically coupled to aresonator.

FIG. 3B is a cross-sectional view of the waveguide-coupled resonatorsystem shown in FIG. 3A and taken along line 3B-3B.

FIG. 3C is a cross-sectional view of an alternative waveguide-coupledresonator system with two waveguides positioned above and verticallycoupled to a resonator.

FIG. 4A is a schematic view of a waveguide-coupled optical resonatorsystem with two waveguides positioned above and below and verticallycoupled to a resonator.

FIG. 4B is a cross-sectional view of the waveguide-coupled resonatorsystem shown in FIG. 4A and taken along line 4B-4B.

FIG. 5 is a graph showing a typical resonance spectrum of a resonatormade from typical semiconductor materials with a 10 μm diameter.

FIG. 6A is a schematic view of a first embodiment of a tunable laser ofthe present invention including a laser diode coupled to awaveguide-coupled optical resonator system with two waveguideshorizontally coupled to an optical resonator.

FIG. 6B is a schematic view of an alternative to the first embodiment ofthe tunable laser of the present invention shown in FIG. 6A.

FIG. 7 is a graph showing the effect of imbalanced coupling on theoutput power of a lossless waveguide-coupled resonator system having asingle optical resonator.

FIG. 8 is a graph showing the improvement in output power result fromthe use of multiple resonators in a lossless waveguide-coupled resonatorsystem having imbalanced coupling.

FIG. 9 is a graph showing the output power of a waveguide-coupledresonator system with optical losses and imbalanced coupling fordifferent numbers of optical resonator, wherein the loss coefficient is6.0 cm⁻¹ in the waveguide and 5.0 cm⁻¹ in the resonator.

FIG. 10 is a graph showing the output power of a lossy waveguide-coupledresonator system with imbalanced coupling for different numbers ofoptical resonator, wherein the loss coefficient in the waveguide is 6.0cm⁻¹ and 2.0 cm⁻¹ in the resonator.

FIG. 11 is a graph showing effective reflectivity as a function of thenumber of resonators in a waveguide-coupled resonator system withimbalanced coupling and different optical loss coefficients.

FIG. 12A is a schematic view of a second embodiment of a tunable laserof the present invention similar to that shown in FIG. 6A except for theuse of multiple optical resonators.

FIG. 12B is a schematic view of an alternative to the second embodimentof a tunable laser of the present invention shown in FIG. 12A andsimilar to that shown in FIG. 6B except for the use multiple opticalresonators.

FIG. 13A is a schematic view of a typical pin wafer structure for anoptical resonator of the present invention based on an InGaAsP/InPmaterial system.

FIG. 13B is a graphical representation of the energy bandgap of the pinwafer structure shown in FIG. 13A.

FIG. 13C is a graphical representation of the refractive index profileof the pin wafer structure shown in FIG. 13A.

FIG. 14 is a graph showing changes in refractive index due to thecarrier effects as a function of injected current and current densityfor a 10 μm diameter resonator based on the pin wafer structure shown inFIG. 13A and a working wavelength of 1.55 μm.

FIG. 15 is a graph showing changes in refractive index changes due toPockels and Franz-Kedysh effects as a function of an electrical fielddue to an applied reversed bias voltage for a 10 micron diameterresonator based on the pin wafer structure shown in FIG. 13A and aworking wavelength of 1.551 μm.

FIG. 16A is a schematic view of a third embodiment of a tunable laser ofthe present invention similar to that shown in FIG. 6A except thewaveguides are vertically coupled to the optical resonator.

FIG. 16B is a schematic view of an alternative to the third embodimentof a tunable laser of the present invention shown in FIG. 16A andsimilar to that shown in FIG. 6B except the waveguides are verticallycoupled to the optical resonator.

FIG. 17A is a schematic view of a fourth embodiment of a tunable laserof the present invention similar to that shown in FIG. 16A except thewaveguides, which are vertically coupled to the optical resonator, arepositioned above and below the resonator.

FIG. 17B is a schematic view of an alternative to the fourth embodimentof a tunable laser of the present invention shown in FIG. 17A andsimilar to that shown in FIG. 16B except the waveguides, which arevertically coupled to the optical resonator, positioned above and belowthe resonator.

FIG. 18A is a schematic view of a fifth embodiment of a tunable laser ofthe present invention similar to that shown in FIG. 16A except for theuse of multiple optical resonators.

FIG. 18B is a schematic view of an alternative to the fifth embodimentof a tunable laser of the present invention shown in FIG. 18A andsimilar to that shown in FIG. 16B except for the use multiple opticalresonators.

FIG. 19A is a schematic view of a sixth embodiment of a tunable laser ofthe present invention similar to that shown in FIG. 17A except for theuse of multiple optical resonators.

FIG. 19B is a schematic view of an alternative to the sixth embodimentof a tunable laser of the present invention shown in FIG. 19A andsimilar to that shown in FIG. 17B except for the use multiple opticalresonators.

FIG. 20A is a schematic view of a seventh embodiment of a tunable laserof the present invention using a monolithic integration process in whichthe laser diode and the waveguide-coupled resonator are grown on thesame substrate.

FIG. 20B is a schematic view of an alternative to the seventh embodimentof a tunable laser shown in FIG. 20A with a vertical couplingconfiguration.

FIG. 20C is a schematic view of another alternative to the seventhembodiment of a tunable laser shown in FIG. 20A with a vertical couplingconfiguration having waveguides positioned above and below theresonator.

FIG. 21A is a schematic view of an eighth embodiment of a tunable laserof the present invention using a monolithic integration process in whichthe laser diode and the waveguide-coupled resonator are grown on thesame substrate and are monolithically integrated with anelectro-absorption modulator.

FIG. 21B is a schematic view of a ninth embodiment of a tunable laser ofthe present invention in which an array of tunable lasers aremonolithically integrated with a mutliplexer which is also based onwaveguide-coupled optical resonators.

FIG. 22A is a schematic view of an alternative to the eighth embodimentof a tunable laser shown in FIG. 21A in which an amplifier ismonolithically integrated into the system adjacent theelectro-absorption modulator.

FIG. 22B is a schematic view of another alternative to the eighthembodiment of a tunable laser shown in FIG. 21A in which amplifiers aremonolithically integrated into the waveguides of the system.

DETAILED DESCRIPTION OF THE INVENTION

Referring in detail to the figures, a wavelength tunable laser of thepresent invention, as shown, combines a semiconductor laser diode with awaveguide-coupled optical resonator. The waveguide-coupled opticalresonator serves as a wavelength selective, external cavity to realize atunable laser by applying voltage or injecting current to change theresonance wavelength of the resonator(s).

Turning to FIG. 1, a multi-layer semiconductor wafer structure of alaser diode 10 of the present invention is shown. The laser diode 10preferably comprises an active or guiding layer 12 sandwiched betweentwo cladding layers 13 and 14 on a substrate 15. The active layer(guiding) 12 is preferably non-doped or insulated and has a higherrefractive index (n) and a smaller bandgap than the two cladding layers13 and 14. The two cladding layers 13 and 14 are preferably highly dopedwith either n-type or p-type doping in order to reduce contactresistance. The substrate 15 is preferably more highly doped with eithern-type or p-type doping. This structure 10 provides good confinement forboth photons and carriers as a result. Electrodes or electrical contacts16 and 17 are formed on the top and bottom of the structure 10 to applyvoltage or inject current. In a preferred construction, the structure 10includes a n-doped substrate 15, a n-doped first cladding layer 13positioned on top of the substrate 15, a non-doped guiding layer 12positioned on top of the first cladding layer 13, a p-doped secondcladding layer 14 positioned on top of the guiding layer 12, a negativeelectrical contact 17 positioned on the bottom of the substrate 15 and apositive electrical contact 16 positioned on top of the second claddinglayer 14.

The wafer structure 10 is preferably cleaved along its crystal planeforming two end facets 18 and 19. The end facets 18 and 19 typically actas mirror surfaces that reflect light beams back and forth within theactive layer 13; wherein the end facets 18 and 19 and active layer 13form a Fabry-Perot (F-P) cavity. However, as discussed in greater detailbelow, when incorporated as part of a tunable laser cavity of thepresent invention, one of the end facets of the laser diode wouldpreferably be coated with an anti-reflection (A.R.) coating. The A.R.coating allows light to be spontaneously emitted from the A.R. coatedend facet when current is injected into the laser diode.

The resonance wavelength of a typical F-P cavity is given by$\begin{matrix}{{\lambda_{q} = \frac{c}{2{qnl}}},\quad{q = 1},{2\quad.\quad.\quad.}} & (1)\end{matrix}$where c is the light speed in free space, n is the refractive index ofthe active medium (layer 12), and l is the cavity length. The opticalloss comes from the absorption of the active medium and the partialreflections at the two end facets. The loss coefficient per round tripfor the light is given by $\begin{matrix}{\alpha = {{\alpha_{s} + \alpha_{m}} = {{\alpha_{s} + \alpha_{m1} + \alpha_{m2}} = {\alpha_{s} + {\frac{1}{2l}\ln\frac{1}{R_{1}R_{2}}}}}}} & (2)\end{matrix}$where α_(s) is the absorption loss coefficient, α_(m) is the mirror losscoefficient due to the partial reflections and R₁, R₂ are the powerreflectivities at the two end facets. At the semiconductor-airinterface, the reflectivity is usually given by$R = {\left( \frac{n - 1}{n + 1} \right)^{2}.}$For III-V semiconductor materials, the refractive index (n) is about3.4, therefore R≈30%. See, e.g., Advanced III-V Compound SemiconductorGrowth, Processing and Devices (Materials Research Society SymposiumProceedings, Vol. 240 (Pearton et al. (Ed.)). When one of the resonancewavelengths of this cavity has a gain in the active medium large enoughto overcome the optical loss incurred over a round trip through thecavity, lasing oscillation may result at this particular wavelength.

For use in optical transmission systems based on fiber optics, theuseful lasing wavelengths, or optical fiber windows, tend to be 1.3 and1.55 μm because of the minimal dispersion losses at 1.3 μm and minimalabsorption losses at 1.55 μm. Thus, in order to emit light at thesewavelengths the laser diode 10 of the present invention is preferablyformed from InP material. However, for other applications the laserdiode may be formed from other material systems. For example, for use inshort distance communications across copper cables, the laser diode maybe formed from an InGaAsP/InP material system, which has a lasingwavelength at about 860 nm.

Turning to FIGS. 2A, 2B, 3A, 3B, 3C, 4A, and 4B, illustrativeembodiments of the waveguide-coupled optical resonators 20, 30 and 40 ofthe present invention are shown. As depicted, the multi-layersemiconductor wafer structures of the waveguide-coupled opticalresonators 20, 30, and 40 include a circular or disk-shaped resonatorcavity 22, 32 and 42 and two adjacent waveguides 24 and 26, 34 and 36,and 44 and 46, respectively, formed as a single chip structure.Preferably, the waveguides, which are either horizontally or verticallycoupled to the resonators through evanescent wave coupling, are taperedin structure, wherein the ends are thicker than a central portion. Thetapered structure of the waveguides enables the waveguide-coupledoptical resonators to take advantage of mode (i.e., wavelength) matchingbetween the waveguides and the resonator at an interaction region 27, 37and 47 of the chip.

Referring to FIGS. 2A and 2B, a horizontally coupled waveguide-coupledresonator 20 is shown to preferably comprise a resonator 22 and firstand second waveguides 24 and 26 formed on top of a highly n-dopedsubstrate 21, wherein the waveguides 24 and 26 are spaced apart from theresonator 22 across predetermined coupling gaps 23 and 25. The resonator22 and waveguides 24 and 26 preferably include a first layer L1, whichis preferably a n-doped cladding layer, positioned on top of thesubstrate 21, a second layer L2, which is preferably a non-doped activeor guiding layer, positioned on top of the first layer L1, and a thirdlayer L3, which is preferably a p-doped cladding layer, positioned ontop of the second layer L2. A positive electrode contact 28 ispositioned on top of the resonator 22 and a negative electrode contact29 is positioned on the bottom of the substrate 21.

The horizontally coupled waveguide-coupled optical resonator 20 may beformed using electron beam (e-beam) lithography and standardsemiconductor fabrication processes, such as plasma-enhanced chemicalvapor deposition (PECVD), reactive ion etching (RIE), and inductivelycoupled plasma (ICP) etching. The coupling between the resonator 22 andthe waveguides 24 and 26 is controlled by varying the size of the gaps23 and 25 between the waveguides 24 and 26 and the resonator 22. The gapsize is preferably small enough to enable evanescent coupling, whichoccurs through the evanescent wave of guided light. The evanescent waveis the tail of the guided light that extends beyond the waveguide layeras the light propagates along the waveguide. The tail decays as thedistance away from the center of the guiding layer increases, andbecomes zero at infinite. If two waveguides are placed sufficientlyclose together, the tail of the light guided in a first waveguide willoverlap the adjacent guiding layer enabling the light signal to becoupled into the second waveguide.

The gap size is also related to the coupling efficiency between thewaveguide and the resonator. Theoretically, the value of the couplingefficiency is not critical in order to have 100% power transfer. Asdiscussed in greater detail below, as long as the coupling is balancedand the resonator is optically lossless, 100% power transfer is possiblewhen the wavelength of the light signal is at resonance, even if thecoupling efficiency is very small. However, in reality, optical lossesdo exist in the resonator. If the coupling efficiency is too small, theoptical power transferred into the resonator will disappear after ittravels a very short distance and, as a result, may not reach the othercoupling or interaction region. Thus, a certain level of couplingefficiency, which is basically determinable by gap size and waveguidestructure, is needed. The smaller the gap size, the larger the couplingefficiency tends to be. A gap size of 0.2 um accompanied by the taperedwaveguide structures of the present invention, tends to provide acoupling efficiency of 2-3%. With current fabrication technologies, suchas e-beam lithography, a gap size as small as 0.1 um may be achieved.Therefore, in order to enable evanescent coupling and sufficientcoupling efficiency, the size of the coupling gaps 23 and 25 ispreferably about 0.1-0.2 microns. As fabrication technologies advance, agap size smaller than 0.1-0.2 microns may be more desirable.

Turning to FIGS. 3A and 3B, a vertical coupling waveguide-coupledoptical resonator 30 is shown to preferably include a resonator 32 andfirst and second waveguides 34 and 36 formed on top of a patternedpolymer wafer 33 positioned on top of a transfer substrate 31. Thewaveguides 34 and 36, which are formed in the same layer, preferablyinclude a first layer L1, which is preferably a non-doped claddinglayer, positioned on opposite sides and on top of the polymer wafer 33and a second layer L2, which is preferably a non-doped guiding or activelayer, positioned on opposite sides of the polymer wafer 33 and on topof the first layer L1. The resonator 32 preferably includes a thirdlayer or separation layer L3, which is preferably a n-doped claddinglayer, positioned on top of the second layer L2 and a top portion of thepolymer wafer 37, a fourth layer L4, which is preferably a non-dopedguiding or core layer, positioned on top of the third layer L3, and afifth layer L5, which is preferably a p-doped cladding layer, positionedon top of the fourth layer L4. A negative electrode contact 39 ispreferably deposited in a recess 35 formed in the top of the polymerwafer 33 adjacent the third n-doped cladding layer L3. A positiveelectrode contact 38 is deposited on top of the fifth p-doped claddinglayer L5.

FIG. 3C provides an alternative vertical coupling geometry wherein thewaveguide-coupled optical resonator 30′ preferably includes first andsecond waveguides 34′ and 36′ positioned on top of a resonator 32′.Specifically, the waveguide-coupled optical resonator 30′ includes apositive electrode contact 39′ deposited on top of a transfer substrate31′. The resonator 32′ preferably includes a first layer L1, which ispreferably a p-doped cladding layer, positioned on top of the positiveelectrode contact 39′, a second layer L2, which is preferably anon-doped guiding or core layer, positioned on top of the first layerL1, and a third or separation layer L3, which is preferably a n-dopedcladding layer, positioned on top of the second layer L2. A negativeelectrode contact 35′ is preferably deposited in a recess 35′ formed inthe top of the third layer L3. The waveguides 34′ and 36′, which areformed in the same layer, preferably include a fourth layer L4, which ispreferably a non-doped guiding layer, positioned on opposite sides ofthe negative electrode 38′ and on top of the third layer L3, and a fifthlayer L5, which is preferably a non-doped cladding layer, positioned ontop of the fourth layer L4.

FIGS. 4A and 4B provide another alternative vertical coupling geometrywherein first and second waveguides 44 and 46 of the waveguide-coupledoptical resonator 40 are formed in different layers, i.e., L2 versus L6;one positioned underneath a resonator 42 and the other positioned abovethe resonator 42. As shown in FIG. 4B, the waveguide-coupled opticalresonator structure 40 comprises a transfer substrate 41 upon which apatterned polymer wafer 43 is positioned. The first waveguide 44includes a first layer L1, which is preferably a non-doped claddinglayer, positioned on top of a side portion of the polymer wafer 43, anda second layer L2, which is preferably a non-doped guiding layer,positioned on top of the first layer L1. The resonator 42 preferablyincludes a third layer or separation layer L3, which is preferably an-doped cladding layer, positioned on top of the second layer L2 and atop portion of the polymer wafer 43, a fourth layer L4, which ispreferably a non-doped guiding or core layer, positioned on top of thethird layer L3, and a fifth or separation layer L5, which is preferablya p-doped cladding layer, positioned on top of the fourth layer L4. Thesecond waveguide 46 preferably includes a sixth layer L6, which ispreferably a non-doped guiding layer, positioned on top of the fifthlayer L5 on a side opposite the first waveguide 44, and a seventh layerL7, which is preferably a non-doped cladding layer, positioned on top ofthe sixth layer L6. A negative electrode contact 49 is preferablydeposited in a recess 45 formed in the polymer wafer 43 adjacent thethird n-doped cladding layer L3. A positive electrode contact 48 isdeposited on top of the fifth p-doped cladding layer L5.

For the vertical coupling waveguide-coupled optical resonators 30, 30′and 40 of the present invention, the coupling tends to be controlled byvarying the thickness of the separation layer, i.e. n-doped claddinglayer L3 in FIGS. 3B and 3C and n-doped and p-doped cladding layers L3and L5 in FIG. 4B, which can be precisely formed through epitaxialgrowth. As a result, the tolerances for vertical coupling geometry tendsto be much higher than the tolerances for horizontal coupling geometry.Because the tolerances for vertical coupling geometry are not as tightas those for horizontal coupling geometry, vertical coupling geometrycan advantageously be realized using traditional photolithographytechniques, which are more efficient than e-beam lithography techniques.

In addition, the layered structures of the vertical coupling geometrymay be formed using polymer, direct or anodic wafer bonding techniques,which enable metal electrode contacts to be deposited on or betweenlayers. Polymer wafer bonding, for instance, enables bonding of twodifferent wafers together by using an organic polymer as theintermediate medium. Compared with other bonding methods, polymer waferbonding is simpler and requires relatively lower processingtemperatures. The polymer may be any commercial polymer used forbonding. Preferably, Benzocyclobutenes (BCBs) polymers, which are arelatively new class of organic polymers, are used in the layerstructures of the present invention.

To form the structures shown in FIGS. 3B and 4B, a layer of SiO₂, about400 nm thick, is first deposited as the etching mask on a patternedepi-wafer 33 and 43. Conventional optical lithography plus standardsemiconductor fabrication processes are then used to make the couplingwaveguide(s) 34, 36 and 44 on the wafer 33 and 43. Metal is thendeposited on the surface of the wafer 33 and 43 to from an Ohmic contact39 and 49. Next, BCB is spun on the bottom of the patterned wafer 33 and43 and a transfer substrate 31 and 41, which is preferably differentfrom the epi-wafer substrate. The patterned wafer 33 and 43 is thenflipped over and placed down onto the transfer substrate 31 and 41. Thetwo wafers are tightened together. Next, the combined wafer is put intoa N₂ filled furnace. The temperature is raised to 250° C. and the waferis baked for about an hour, and then allowed to cool down. After beingfully cured, the BCB layer acts as glue and can bond the two waferstogether very tightly. The thickness of the BCB layer is chosen to bethick enough to prevent optical leakage into the transfer substrate 31and 41. However, if it is too thick, it is difficult to get a good facetquality after cleaving. After the wafer is removed from the furnace, theepi-wafer's substrate is removed using selective wet etching. Theresonator 32 and 42 is then fabricated, using standard semiconductorfabricating processes, on the wafer 33 and 43 and is well-aligned withthe coupling waveguides 34, 36 and 44. Lastly, metal is deposited on thesurface of the resonator 32 and 42 to form an Ohmic contact 38 and 48.If the coupling waveguides 44 and 46 are on different sides of theresonator 42, the second coupling waveguide 46 is then fabricated on theresonator 42 and is well aligned to the resonator 42.

Referring to FIG. 3C, the layered structure is preferably formed bydirect or anodic wafer bonding methods. Direct wafer bonding utilizessurface attraction forces by putting two ultra-cleaned wafer surfacestogether with a certain pressure and/or high temperature annealingprocess. Anodic wafer bonding basically occurs by applying an electricalfield across the two wafers.

In operation, light beams tend to propagate inside the resonators 22, 32and 42 along the circumference of the resonators 22, 32 and 42 by totalreflections in what is commonly referred to as the “Whispering-Gallery”mode. Such a propagation mode usually has a very high Q value due to thestrong mode confinement and low optical loss in the cavity. The mode Qvalue describes the optical mode loss in a cavity, wherein a high Qvalue corresponds to a lower loss, and is defined as $\begin{matrix}{Q = \frac{\lambda_{res}}{\delta\lambda}} & (3)\end{matrix}$where λ_(res) is the resonance wavelength of the resonator, δλ is thefull resonance linewidth at half-maximum resonance. FIG. 5 shows atypical resonance spectrum of a resonator with a 10 μm diameter madefrom typical semiconductor materials corresponding to a center workingwavelength of 1500 nm. From FIG. 5, we can see that the spectrum has afree spectral range (FSR) of 20 nm and the Q value is about 5000, for aresonance wavelength of about 1550 nm and a resonance linewidth of about0.31 nm. The FSR describes the separation between two adjacentresonances. In wavelength domain, FSR is given by $\begin{matrix}{{\Delta\lambda} \approx \frac{\lambda_{m}^{2}}{2{\pi{Rn}}}} & (4)\end{matrix}$where λ_(m) is the center-working wavelength and R is the resonatorradius. To increase the FSR, smaller resonators are preferred.

With coupling waveguides, the optical resonator can have versatilefunctions. However, the main function of the waveguide-coupled opticalresonators 20, 30 and 40 of the present invention is as a wavelengthselector. Referring to FIGS. 2A, 3A and 4A, input light beams withdifferent wavelengths λ₁, λ₂, λ₃, . . . enter the first waveguide 24, 34and 44 and are coupled from the first waveguide 24, 34 and 44 into theresonator 22, 32 and 42 either horizontally or vertically at theinteraction region 27, 37 and 47 through the evanescent wave (i.e.,evanescent coupling) of the light beam when the wavelength of the lightbeam is at a resonance wavelength or frequency of the resonator 22, 32and 42; for instance, λ₁. The light beam with wavelength λ₁ thencirculates inside the resonator 22, 32 and 42, and builds up to largeintensities. At the same time, it is coupled out of the resonator 22, 32and 42 into the second waveguide 26, 36 and 46 through the evanescentwave of the light beam at the interaction region 27, 37 and 47. The restof the input light signals, i.e., λ₂, λ₃, . . . , just pass through andexit the other end of the first waveguide 24, 34 and 44. Resonancewavelength tuning can easily be achieved by either applying a voltage orinjecting current to the resonator 22, 32 and 42. In this way, theoptical power associated with a particular wavelength can be transferredfrom one side of the waveguide-coupled optical resonator 20, 30 and 40to the other side. In an ideal case, 100% of the optical powerassociated with a selected wavelength is transferred from one side ofthe waveguide-coupled optical resonator to the other. Realization of100% power transfer, however, tends to require that the following threeconditions be satisfied:

-   -   (1) The wavelength is at resonance;    -   (2) There is no optical loss in the resonator cavity; and    -   (3) The coupling on each side should be equal, i.e., the        coupling is balanced, and lossless.

Referring to FIG. 6A, a first illustrative embodiment of a wavelengthtunable laser 100 a of the present invention is shown to include asemiconductor laser diode 110, with opposing end facets 118 and 119,that is optically coupled through a coupling lens 111, having a highnumeric aperture (N.A.), to a waveguide-coupled optical resonator 120.The laser diode 110, which is formed from broad gain spectrum material,has substantially the same structure as the laser diode 10 shown in FIG.1 with the exception of the end facet 119 facing the waveguide-coupleresonator 120 being coated with an A.R. coating. The waveguide-coupledresonator 120, which has substantially the same structure as thewaveguide-coupled resonator 20 shown in FIGS. 2A and 2B, includes acircular or disk-shaped resonator cavity 122 with an electrode contact128 shown positioned on top of the resonator 122. An additionalelectrode contact (not shown) may be positioned below the resonator 122as shown in FIG. 2B. The resonator 122 is spaced apart from first andsecond waveguides 124 and 126 across coupling gaps 123 and 125. Thewaveguides 124 and 126, which preferably include a tapered structure,are positioned in parallel orientation on opposite sides of theresonator 122. The tunable laser 100 a further includes a collimatedlens 152 and a mirror 150, preferably with 100% reflectivity, positionedadjacent to one end facet 129 of the second waveguide 126.

In operation, current is applied to the laser diode 110 at a level belowthe lasing threshold of the laser diode 110 to generate spontaneouslight emissions from the A.R. coated end facet 119 of the laser diode110. The light emissions are coupled into the first waveguide 124 of thewaveguide-coupled resonator 120 through the coupling lens 111. The lightpropagates through the first waveguide 124 and reaches the interactionarea 127 of the first waveguide 124 and the resonator 122. Light havinga wavelength associated with a resonance frequency of the resonator 122is horizontally coupled into the resonator 122 through evanescentcoupling across the first coupling gap 123. As the light propagateswithin the resonator 122, the light is horizontally coupled into thesecond waveguide 126 through evanescent coupling across the secondcoupling gap 125 at the interaction area 127 of the second waveguide 126and the resonator 122. Thus, a certain portion of the input opticalpower from the laser diode 110 is transferred from the first couplingwaveguide 124 to the second coupling waveguide 126.

The collimated lens 152 right after the end facet 129 of the secondwaveguide 126 converts the output of the second waveguide 126 intoparallel light beams. The mirror 150 reflects the light or optical powerback into the second waveguide 126. Alternatively, as shown in FIG. 6B,the light may be reflected back into the system by a high reflection(H.R.) coating 154 applied to the end facet 129 of the second waveguide126. The use of a H.R. coating advantageously eliminates the need foradditional optical elements such as a mirror and lens.

The reflected light or optical power propagates back through thewaveguide-coupled resonator 120, i.e., the reflected light propagatesthrough the second waveguide 126 to the interaction area 127 where it iscoupled into the resonator 122 and, as it propagates within theresonator 122, it is coupled into the first waveguide 124 at theinteraction area 127. From the first waveguide 124, the light is coupledback into the laser diode 110. Thus, a closed optical cavity is formedbetween the laser diode 110 and the waveguide-coupled optical resonator120.

By increasing the current injected into the laser diode 110 until thelight associated with the particular resonance frequency selected by theoptical resonator 122 has enough gain to compensate or overcome theoptical losses encountered during the light's propagation through theclosed cavity, the light becomes lasing and is output from the other endfacet 118 of the laser diode 110. Further increases of the currentinjected into the laser diode 110 will increase the lasing output powerof the tunable laser system 100 a and 100 b of the present invention.The level of the injected current, however, remains below the lasingthreshold for the laser diode, but above the lasing threshold of thetunable laser system.

In order to tune the lasing frequency of the tunable laser system 100 aand 100 b, voltage or current may be applied to the optical resonator122. As discussed in detail below, the applied voltage or currentchanges the refractive index of the material in the resonator 122 due toelectro-optical effects or carrier effects, which in turn changes theresonance frequency of the resonator 122. Changing the resonancefrequency of the resonator 122, changes the lasing frequency(wavelength) of the tunable laser system 100 a and 100 b.Advantageously, the lasing frequency (wavelength) may be continuouslychanged or tuned by continuously changing the voltage or current appliedto the resonator 122. Tuning speeds tend to be on the order ofnanoseconds (e.g., 1-100 ns) or faster as a result.

The waveguide-coupled resonator 120 along with the mirror 150 or H.R.coating 154 provide an effective reflection for the laser diode 110 thatis similar to the reflection provided by the end facets of aconventional laser diode in typical use, and may be equivalent to aneffective facet mirror. If P_(i) is the power of the light coupled intothe waveguide-coupled resonator 120 and mirror 150 system, P_(o) is theoutput power of the light after being reflected by the system. Effectivereflectivity is defined as $\begin{matrix}{R_{eff} = \frac{P_{o}}{P_{i}}} & (5)\end{matrix}$

Preferably, effective reflectivity R_(eff) of the system is comparableto the effective reflectivity at the end facets of a conventional laserdiode, which is typically about 30%. However, effective reflectivityR_(eff) is dependent upon how much power can be transferred between thetwo waveguides 124 and 126 of the system, assuming that the light can befully reflected back into the system. In an ideal case, the threeconditions for full power transfer mentioned above are satisfied. Inreality, however, any fabrication error or material defect may cause thecoupling between the waveguides 124 and 126 and the resonator 122 to beimbalanced and may introduce optical loss into the system.

Assuming that there is no optical loss in the waveguides 124 and 126 andthe resonator 122, R_(eff)=1 tends to be unachievable when the couplingon each side is not balanced, i.e., c1≠c2, where c1 and c2 are thecoupling coefficients between the resonator 122 and waveguides 124 and126 across the coupling gaps 123 and 125. This effect is illustrated inFIG. 7. As depicted in FIG. 7, 100% power transfer tends to occur whenc1=c2, but not when c1≠c2. Furthermore, the larger the imbalance, thelower the percentage of power transferred. For example, if c1=0.02,c2=0.005, then there is only about 60% power transferred even with nooptical loss in the system. For all cases shown in FIG. 7, the radius ofthe resonator 122 is 5 μm, the coupling waveguides 124 and 126 aretapered down to about 0.5 μm adjacent the coupling or interaction region127, and the refractive index for the resonator 122 and waveguides 124and 126 is 3.4.

As shown in FIG. 8, the use of multiple resonators instead of a singleresonator tends to improve system performance. FIG. 8 shows the resultsfor lossless systems with imbalanced coupling, i.e., c1=0.02, c2=0.005,and different numbers of resonators (N), with all other parametersremaining the same as for the single resonator case depicted in FIG. 7.When N=1, the percentage of power transferred is about 60%. When N=2,the percentage of power transferred increases to about 95%, assuming inthis case a distance between two adjacent resonators of preferably about24 μm, which ensures constructive interference between the resonators.The preferred distance between resonators to ensure constructiveinterference may be determined by calculating the phase differencebetween the two adjacent resonators, which should be equal to 2mπ, m=0,1, 2, . . . , where m is an integer. When N=4, power transfer approaches100%. This is not surprising if the system is thought of as a grating.The more resonators, the higher the order of the grating. Theimprovement of the performance of the system as the number of resonatorsis increased is mainly due to the constructive interference among theresonators. This can be verified by the change in the response spectrumshape, which has a much flatter passband and a faster rolloff.

In reality, however, such a system would include optical losses. Thepercentage of power transferred for a system similar to the systemdepicted in FIG. 8, except that loss coefficients in the resonators ofα_(RES)=5.0 cm⁻¹ and in the coupling waveguides of α_(WG)=6.0 cm⁻¹ areassumed, is shown if FIG. 9. As shown, the percentage of powertransferred for a single resonator drops to about 38%. The use of tworesonators, however, improves the percentage of power transferred toabout 50%. A further increase in the number of resonators, however, doesnot tend to significantly improve the percentage of power transferred.For example, where N=3, 4, 5, the percentage of power transferred isvery close to that of N=2. The lack of improvement in power transfer dueto the use of multiple resonators is because the beneficial effect ofthe multiple resonators tends to be offset by an increase in opticalloss due to the existence of more resonators.

In order to fully utilize the beneficial effect that multiple resonatorsmay have on the percentage of power transfer, the optical loss due tothe resonators is preferably minimized. The percentage of powertransferred for a system similar to the system depicted in FIG. 9,except that the loss coefficient in the resonators is decreased to 2.0cm⁻¹, is shown if FIG. 10. With a lower loss coefficient for theresonator, the percentage of power transferred in a single resonatorsystem tends to be about 50%. The percentage of power transferred tendsto increase to about 70% for N=2 and 75% for N=3, 4 and 5. Compared toFIG. 9, the improvement is significant. Therefore, having low lossresonators in the system is very advantageous in that it results in highreflectivity for the laser diode. However, as the results for N=3, 4 and5 indicate, it is not necessary to have as many resonators as possible.As FIGS. 9 and 10 indicate, two (N=2) resonators tends to be enough tosufficiently improve the percentage of power transfer in a system. As aresult, a tunable laser device of the present invention mayadvantageously be very compact.

Turning to FIG. 11, effective reflectivity is shown as a function of thenumber of resonators in a system with imbalanced coupling and differentoptical loss coefficients. As FIG. 11 indicates, when N>3 the change ineffective reflectivity is minimal. In order to have an effectivereflectivity R_(eff) of about 30% or higher, it is preferable that theloss coefficient in the resonator(s) is less than about 3 cm⁻¹, whichmainly depends on the fabrication processes.

Referring to FIGS. 12A and 12B, additional illustrative embodiments ofthe tunable laser 101 a and 101 b of the present invention are shown toinclude multiple resonators 122 _(i), 122 _(ii), and 122 _(iii) insteadof a single resonator. With the exception of multiple resonators 122_(i), 122 _(ii), and 122 _(iii), the structure of these tunable lasers101 a and 101 b correspond to the structure of the tunable lasers 100 aand 100 b shown in FIGS. 6A and 6B with like elements identified withthe same element numerals. Electrode contacts 128 _(i), 128 _(ii), and128 _(iii) are shown positioned on top of the resonators 122 _(i), 122_(ii), and 122 _(iii). A second set of electrode contacts (not shown)are positioned below the resonators 122 _(i), 122 _(ii), and 122 _(iii)as shown in FIG. 2B. Although FIGS. 12A and 12B show tunable lasers 101a and 101 b utilizing three resonators 122 _(i), 122 _(ii), and 122_(iii), one skilled in the art would understand that a tunable laser ofthe present invention may comprise two or more than three resonators.However, according to the analysis presented above in regard to FIGS. 9,10 and 11, two or three resonators may be sufficient to improve thepercentage of power transferred to a desirable level.

As noted above, wavelength tuning is realized in the tunable lasers ofthe present invention by changing the resonance wavelength, i.e.,frequency, of the optical resonator. There are basically two mainalternative methods of electrically changing the refractive index of asemiconductor, of which the optical resonator of the present inventionis composed, which results in a change in resonance frequency of theresonator. The first method utilizes the electro-optic effects thatresult when a voltage is applied to the resonator. The second methodutilizes the carrier effects resulting from injecting current into theresonator. These effects are described below in regard to a typicalwafer structure for an optical resonator 160 of the present inventionshown in FIG. 13A.

The wafer structure of the optical resonator 160 shown in FIG. 13A isbasically the same as the wafer structure of the semiconductor laserdiode 10 shown in FIG. 1A, which provides the possibility for monolithicintegration of the components of the tunable laser system of presentinvention. The structure preferably includes a first cladding layer 162,which is preferably formed from highly n-doped InP material, a secondcladding layer 164, which is preferably formed from highly p-doped InPmaterial, and a core or guiding layer 163, which is preferably formedfrom non-doped InGaAsP material, sandwiched between the two claddinglayers 162 and 164. Where monolithic integration is unnecessary, theresonator core may be formed from other materials such as GaAs/AlGaAs,SiOxSi, polymers, and the like. All of the layers are formed on top of asubstrate 161, which is preferably formed from more highly n-doped InPmaterial. The cladding layers 162 and 164 are preferably about 1.0 μmthick, while the guiding layer 163 is preferably about 0.4 μm thick.FIGS. 13B and 13C illustrate the energy bandgap structure and therefractive index distribution, respectively, for the pin structure shownin FIG. 13A.

Basically, both applying voltage and injecting current will induce achange in the effective refractive index of the optical resonator 160,which in turn changes the resonance spectrum of the resonator 160. Inthe resonator 160 of the present invention, whispering-gallery modes(WGMs) dominate. These WGMs usually have high Q values which are givenby equation (3). The change in refractive index needed for a halfresonance linewidth shift of δλ/2 may be estimated as follows. Theresonance wavelength of λ₁ may be approximated from $\begin{matrix}{{\frac{2{\pi L}_{c}}{\lambda_{1}}n} = {2{m\pi}}} & \left( {6a} \right)\end{matrix}$which, for a refractive index change of Δn, becomes $\begin{matrix}{{\frac{2{\pi L}_{c}}{\lambda_{1} + {{\delta\lambda}/2}}\left( {n + {\Delta n}} \right)} = {2{m\pi}}} & \left( {6b} \right)\end{matrix}$where L_(c) is the circumference of the microdisk resonator, n is theeffective refractive index, and m is the azimuthal index which describesthe mode distribution along the disk circumference. Combining equations(6a) and (6b) and solving for Δn, the resulting equation becomesΔn=mλ ₁/(2L _(c) Q)  (7)

Assuming a 10 μm-diameter microdisk resonator, where m=60, λ₁=155 μm,L_(c)=πd≈31.4 μm, and Q≈200, then from equation (7) Δn=7.4×10⁻³. Thisrefractive index change enables a shift of the resonance wavelength ofabout δλ/2≈3.8 nm. However, as discussed above such a small Q value isundesirable because the losses associated with the optical resonatorwould be too large. Preferably, the resonators of the present inventionhave high Q values, which, as noted in the analysis above,advantageously results in a system having high effective reflectivity(see FIGS. 9, 10 and 11). Assuming Q≈5000, from equation (7),Δn=2.96×10⁻⁴. Such a change in refractive index causes a resonance shiftof δλ/2=λ_(r)(2Q)=0.16 nm.

The tuning range of the resonator, however, is determined by the freespectral range (FSR), i.e., Δλ. The FSR of a 10 μm-diameter microdisk isabout 20 nm and should satisfy the following equation $\begin{matrix}{{\frac{2{\pi L}_{c}}{\lambda_{1} + {\Delta\lambda}}n} = {{2{m\pi}} - {2\pi}}} & {(8)\quad}\end{matrix}$which, for a shift of the resonance by Δλ, becomes $\begin{matrix}{{\frac{2{\pi L}_{c}}{\lambda_{1} + {\Delta\lambda}}\left( {n + {\Delta n}} \right)} = {2{m\pi}}} & (9)\end{matrix}$Combining equation (8) and equation (9) and solving for Δn, results in$\begin{matrix}{{\Delta\quad n} = {\frac{\lambda_{1} + {\Delta\lambda}}{L_{c}} = 0.05}} & (10)\end{matrix}$Δn is positive because a red shift of resonance is assumed. The carriereffects and electro-optic effects are calculated as follows to estimatehow much current or voltage is needed for such a change in refractiveindex.

There are basically three carrier effects that are responsible for therefractive index change, i.e., band filling, bandgap shrinkage and freecarrier absorption. FIG. 14 shows refractive index change Δn asfunctions of injected current density and injected current for a 10μm-diameter microdisk resonator with a wafer structure as described inFIG. 13A while considering all the carrier-induced effects, i.e.,band-filling, bandgap shrinkage and free carrier absorption. For acarrier concentration of less than 10¹⁶, bandgap shrinkage effect can beneglected as the interparticle spacing is too large (i.e., χ<χ_(cr),where χ is the carrier concentration) and bandfilling dominates, whichyields a negative Δn. However, bandgap shrinkage effect becomesimportant over the range 10¹⁶<χ<10¹⁷ and approximately cancels thebandfilling and free carrier absorption effects. For higher carrierconcentration, bandfilling and free carrier absorption effects dominateand give a large negative Δn. From FIG. 14, in order to achieve arefractive index change of 0.05, a 1.8×10⁶ mA/cm² current density or1.33 mA current for the 10 μm-diameter microdisk resonator is needed.Note that in the calculations the injected carrier density is obtainedby $\begin{matrix}{J = \frac{qdN}{\tau}} & (11)\end{matrix}$where τ≈1 ns is the carrier lifetime, q is electron charge, N is thecarrier concentration and d=0.4 μm is the microdisk core thickness. FromFIG. 14, for a refractive index change up to 10⁻¹, a current density ofonly about 3.4×10⁶ mA/cm² is needed. Since the size of microdiskresonators could be a few microns, the driving current will be about afew mAs. Such a small driving current is one of the advantages of thetunable lasers of the present invention.

When a reversed bias voltage is applied to the pin junctions, there willbe an electrical field across the non-doped region. Two majorelectro-optic effects associated with this electrical field will changethe refractive index of the material: (1) Linear electro-optic (Pockels)effect; and (2) Electrorefractive (Franz-Keldysh) effect. The Pockeleffect is polarization dependent and dominates at relatively lowelectrical field intensity. The Franz-Keldysh effect will have the majorcontribution when the working wavelength is very close to the bandgapand at relatively high electrical field. With the wafer structuredescribed in FIG. 13, the refractive index change caused by the twoeffects is shown in FIG. 15. From FIG. 15, the Pockel effect tends to belarger than the Franz-Keldysh effect when the electrical fieldE<2×10⁴V/cm. However, the Franz-Keldysh effect tends to begin todominate at higher field, but will decrease if the field continues toincrease. In order to have a refractive index change on the order of0.01, the required electrical field tends to be about 10⁶ V/cm. If thethickness of the non-doped layer is 0.4 μm, the required voltage tendsto be about 40V (assuming the voltages dropped across the highly dopedlayers and the interface between the electrode and the contact layers(Ohmic contact) are relatively negligible.) Compared with the carriereffects, the electro-optic effects tend to be less efficient.

The foregoing description of the present invention has illustrated thedetails of a wavelength tunable laser by combining a semiconductor laserdiode and tunable waveguide-coupled optical resonators as the closedwavelength selective external cavity. It will be appreciated by those ofskill in the art that the waveguides of the tunable waveguide-coupledoptical resonators of the present invention can be coupled to theoptical resonator either horizontally (see FIGS. 6A, 6B, 12A and 12B) orvertically. FIGS. 16A and 16B, for example, illustrate other embodiments102 a and 102 b of the tunable laser of the present invention that arevery similar to the tunable lasers 100 a and 100 b shown in FIGS. 6A and6B, respectively, with the exception of the waveguides 134 and 136 beingvertically, instead of horizontally, coupled to the resonator 132. Moreparticularly, the tunable lasers 102 a and 102 b of the presentinvention include a semiconductor laser diode 110, with opposing endfacets 118 and 119, optically coupled through a high N.A. coupling lens111 to a waveguide-coupled resonator 130. The laser diode 110 hassubstantially the same structure as the laser diode 10 shown in FIG. 1with the exception of the end facet 119 that is adjacent thewaveguide-coupled optical resonator 130 being coated with an A.R.coating. The waveguide-coupled resonator 130, which has substantiallythe same structure as the waveguide-coupled resonator 30 shown in FIGS.3A and 3B, includes a circular or disk-shaped resonator cavity 132 withan electrode contact 138 deposited on top of the resonator 132. Anadditional electrode contact (not shown) may be deposited below theresonator 132 as shown in FIG. 3B. The resonator 132 is positioned abovethe first and second waveguides 134 and 136 and separated therefrom by aseparation layer. The waveguides 134 and 136, which preferably include atapered structure, are positioned in parallel orientation in the samelayer. The tunable laser 102 a further includes a collimated lens 152and a mirror 150, positioned adjacent to an end facet 139 of the secondwaveguide 136 or, alternatively, as shown in FIG. 16B, the tunable laser102 b includes a H.R. coating 154 applied to the end facet 139 of thesecond waveguide 136.

In operation, current is applied to the laser diode 110 at a level belowthe lasing threshold of the laser diode 110 to generate spontaneouslight emissions from the A.R. coated end facet 119 of the laser diode110. The light emissions are coupled into the first waveguide 134 of thewaveguide-coupled resonator 130 through the coupling lens 111. The lightpropagates through the first waveguide 134 and reaches the interactionarea 137 of the first waveguide 134 and the resonator 132. Light havinga wavelength associated with a resonance frequency of the resonator 132is vertically coupled into the resonator 132 through evanescent couplingacross the separation layer. As the light propagates within theresonator 132, the light is vertically coupled into the second waveguide136 through evanescent coupling across the separation layer at theinteraction area 137 of the second waveguide 136 and the resonator 132.Thus, a certain portion of the input optical power from the laser diode110 is transferred from the first coupling waveguide 134 to the secondcoupling waveguide 136.

In the tunable laser 102 a shown in FIG. 16A, the collimated lens 152right after the end facet 139 of the second waveguide 136 converts theoutput of the second waveguide 136 into parallel light beams. The mirror150 reflects the light or optical power back into the second waveguide136. In the tunable laser 102 b shown in FIG. 16B, the H.R. coating 154on the end facet 139 of the second waveguide 136 reflects the light oroptical power back into the second waveguide 136. The reflected light oroptical power propagates back through the waveguide-coupled resonator130, i.e., the reflected light propagates through the second waveguide136 to the interaction area 137 where it is coupled into the resonator132 and, as it propagates within the resonator 132, it is coupled intothe first waveguide 134 at the interaction area 137. From the firstwaveguide 134, the light is coupled back into the laser diode 110. Thus,a closed optical cavity is formed between the laser diode 110 and thewaveguide-coupled optical resonator 130. Lasing and tuning of the lasingwavelength are accomplished as discussed above.

Referring to FIGS. 18A and 18B, additional illustrative embodiments 104a and 104 b of the tunable laser of the present invention are shown toinclude multiple resonators 132 _(i), 132 _(ii), and 132 _(iii) insteadof a single resonator. With the exception of multiple resonators 132_(i), 132 _(ii), and 132 _(iii), the structure of the tunable lasers 104a and 104 b correspond to the structure of the tunable lasers 102 a and102 b shown in FIGS. 16A and 16B with like elements identified with thesame element numeral. Electrode contacts 138 _(i), 138 _(ii), and 138_(iii) are shown positioned on top of the resonators 132 _(i), 132_(ii), and 132 _(iii). A second set of electrical contacts (not shown)are positioned below the resonators 132 _(i), 132 _(ii), and 132 _(iii)as shown in FIG. 3B.

The tunable lasers 102 a, 102 b, 104 a and 104 b, as depicted in FIGS.16A, 16B, 18A and 18B, respectively, include coupling waveguides thatare formed in the same layer of the device. Those of skill in the artwill appreciate that the coupling waveguides may be formed in differentlayers as shown in FIGS. 17A, 17B, 19A and FIG. 19B.

Referring to FIGS. 17A and 17B, the tunable lasers 103 a and 103 b ofthe present invention are shown to include a semiconductor laser diode110, with opposing end facets 118 and 119, optically coupled through ahigh N.A. coupling lens 111 to a waveguide-coupled resonator 140. Thelaser diode 110 has substantially the same structure as the laser diode10 shown in FIG. 1 with the exception of the end facet 119 that isadjacent the waveguide-coupled optical resonator 140 being coated withan A.R. coating. The waveguide-coupled resonator 140, which hassubstantially the same structure as the waveguide-coupled resonator 40shown in FIGS. 4A and 4B, includes a circular or disk-shaped resonatorcavity 142 with an electrode contact 148 shown positioned on top of theresonator 142. An additional electrode contact (not shown) may bepositioned below the resonator 142 as shown in FIG. 4B. The resonator142 is positioned above the first waveguide 144 and below the secondwaveguide 146 and separated therefrom by separation layers. Thewaveguides 144 and 146, which preferably include a tapered structure,are positioned in parallel orientation in different layers. The tunablelaser, as shown in FIG. 17A, further includes a collimated lens 152 anda mirror 150, positioned adjacent to an end facet 149 of the secondwaveguide 146 or, alternatively, as shown in FIG. 17B, a H.R. coating154 applied to the end facet 149 of the second waveguide 146.

In operation, current is applied to the laser diode 110 at a level belowthe lasing threshold of the laser diode 110 to generate spontaneouslight emissions from the A.R. coated end facet 119 of the laser diode110. The light emissions are coupled into the first waveguide 144 of thewaveguide-coupled resonator 140 through the coupling lens 111. The lightpropagates through the first waveguide 144 and reaches the interactionarea 147 of the first waveguide 144 and the resonator 142. Light havinga wavelength associated with a resonance frequency of the resonator 142is vertically coupled into the resonator 142 through evanescent couplingacross the first separation layer. As the light propagates within theresonator 142, the light is vertically coupled into the second waveguide146 through evanescent coupling across the second separation layer atthe interaction area 147 of the second waveguide 146 and the resonator142. Thus, a certain portion of the input optical power from the laserdiode 110 is transferred from the first coupling waveguide 144 to thesecond coupling waveguide 146.

In the tunable laser 103 a shown in FIG. 17A, the collimated lens 152right after the end facet 149 of the second waveguide 146 converts theoutput of the second waveguide 146 into parallel light beams. The mirror150 reflects the light or optical power back into the second waveguide146. In the tunable laser 103 b shown in FIG. 17B, the H.R. coating 154on the end facet 149 of the second waveguide 146 reflects the light oroptical power back into the second waveguide 146. The reflected light oroptical power propagates back through the waveguide-coupled resonator140, i.e., the reflected light propagates through the second waveguide146 to the interaction area 147 where it is coupled into the resonator142 and, as it propagates within the resonator 142, it is coupled intothe first waveguide 144 at the interaction area 147. From the firstwaveguide 144, the light is coupled back into the laser diode 110. Thus,a closed optical cavity is formed between the laser diode 110 and thewaveguide-coupled optical resonator 140. Lasing and tuning of the lasingwavelength are accomplished as discussed above.

Referring to FIGS. 19A and 19B, additional illustrative embodiments 105a and 105 b of the tunable laser of the present invention are shown toinclude multiple resonators 142 _(i), 142 _(ii), and 142 _(iii) insteadof a single resonator. With the exception of multiple resonators 142_(i), 142 _(ii), and 142 _(iii), the structure of these tunable lasers105 a and 105 b correspond to the structure of the tunable lasers 103 aand 103 b shown in FIGS. 17A and 17B with like elements identified withthe same element numeral. Electrode contacts 148 _(i), 148 _(ii), and148 _(iii) are shown positioned on top of the resonators 142 _(i), 142_(ii), and 142 _(iii). A second set of electrode contacts (not shown)are positioned below the resonators 142 _(i), 142 _(ii), and 142 _(iii)as shown in FIG. 4B.

Monolithic integrated tunable lasers 106, 107, and 108 of the presentinvention are shown in FIGS. 20A, 20B and 20C, respectively, to includea semiconductor laser diode 110 integrated with a waveguide-coupledresonator 120, 130 and 140 on the same substrate 121, 131 and 141.Monolithic integration may be accomplished using regrowth methods, i.e.,the laser diode layers 110 are grown first on the substrate 121, 131 and141 and then the chip is made with the standard semiconductor waferfabrication processes. Some areas on the substrate are etched away andthe layers for the waveguide-coupled resonator 120, 130 and 140 are thengrown. Again, standard semiconductor fabrication processes are used todefine the waveguide-coupled resonator 120, 130 and 140. It isadvantageous, however, to have one of the end facets of the laser diode110 well aligned and coupled to the first waveguide 124, 134 and 144 ofthe waveguide-couple resonator 120, 130 and 140. The butt-joint couplingmethod, which has been demonstrated in narrow-linewidth DBR lasers andintegrated DFB laser/electroabsorption modulators, results in nearly100% coupling efficiency. Preferably, high reflection coating 154 isdeposited on one end facet of the second waveguide 126, 136 and 146. Thereflectivity for such coatings may be more than 90%. Thewaveguide-coupled resonator 120, 130 and 140 may also comprise multipleresonators to improve the performance as discussed above.

The tunable laser of the present invention is also adaptable to moreadvanced monolithic integrations, i.e., more optical devices can beintegrated with the tunable laser into the same substrate and, thus,more complex functions can be realized. For example, as shown in FIG.21A, an optical device 200, such as a tunable transmitter ortransponder, may comprise an electro-absorption (EA) modulator 256fabricated on the same substrate 231 right after the tunable laser 201wherein the output from the tunable laser 201 can be modulated. Thetunable laser 201 includes a laser diode 210 coupled to awaveguide-coupled resonator 230 comprising a resonator 232 coupled tofirst and second waveguides 234 and 236 with a H.R. coating applied toan end facet of the second waveguide 236.

Another example, as shown in FIG. 21B, is the use of a waveguide-coupledresonator 360 to multiplex the outputs from multiple tunable lasers 301,302 and 303 to form a waveguide bus 300. Preferably, thewaveguide-couple resonator 360 comprises separate input waveguides 361,362 and 363 coupled to the outputs from separate tunable lasers 301, 302and 303. The tunable lasers comprise a laser diode 310 coupled to awaveguide-coupled optical resonator comprising first and secondwaveguides 334 and 336 coupled to an optical resonator 332. Lightentering the input waveguides 361, 362 and 363 from the tunable lasers301, 302 and 303 having wavelengths equal to a resonance frequency ofthe resonators 365, 366 and 367 is transferred to the resonators andonto the output waveguide 364 by evanescent coupling.

In addition, the optical devices of the present invention may be madevirtually lossless by integrating amplifiers into the devices. As shownin FIGS. 22A and 22B, the optical device 200 shown in FIG. 21A may bemodified by integrating an external gain section 280 into the opticaldevice 200 a adjacent to the EA modulator 256 or by making part of thewaveguides 234 and 236 of the optical device 200 b into amplifiers 281and 282. By injecting current into these gain sections, additionaloptical gain is provided for the light and lower threshold lasing can beachieved.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown in thedrawings and are herein described in detail. It should be understood,however, that the invention is not to be limited to the particular formdisclosed, but to the contrary, the invention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the appended claims.

1. In a tunable laser system comprising a laser and a resonator havingan adjustable refractive index induced tunable resonance wavelength, amethod for tuning the wavelength of the tunable laser system comprising:adjusting the refractive index of the resonator by applying anelectrical signal to the resonator such that the resonance wavelength ofthe resonance corresponds to a selected wavelength; coupling lightemissions from the laser into the resonator through evanescent coupling,whereby substantially only the light emissions having a wavelength atthe resonance wavelength of the resonator are coupled into theresonator; and coupling light at the resonance frequency of theresonator from the resonator back into the laser.
 2. The method of claim1, wherein the electrical signal comprises a voltage.
 3. The method ofclaim 1, wherein the electrical signal comprises a current.
 4. Themethod of claim 1, wherein the laser comprises a laser diode.
 5. Themethod of claim 4, further comprising applying a current to the laserdiode below a lasing threshold of the laser diode to generate the lightemissions.
 6. The method of claim 5, further comprising applying enoughcurrent to the laser diode so that light at the resonance frequency ofthe resonator has enough gain to overcome optical losses associated withthe system.
 7. The method of claim 6, wherein the laser diode hasopposing first and second end facets, the light emissions are emittedfrom the second end facet of the laser diode, light at the resonancefrequency of the resonator is coupled from the resonator back into thelaser diode through the second end facet, and the method furthercomprises increasing the current applied to the laser diode until lightat the resonance frequency of the resonator becomes lasing and isoutputted from the first end facet of the laser diode.
 8. The method ofclaim 1, further comprising: coupling light at the resonance frequencyof the resonator from the resonator into a waveguide; reflecting thelight coupled into the waveguide back toward the resonator; and couplingthe reflected light from the waveguide back into the resonator.
 9. Themethod of claim 8, wherein the light in the waveguide is reflected usinga mirror.
 10. The method of claim 8, wherein the light in the waveguideis reflected using a reflective coating on the waveguide.
 11. The methodof claim 8, wherein light is coupled into the waveguide throughevanescent coupling.
 12. The method of claim 1, further comprisingadjusting the resonance frequency of the resonator to another selectedfrequency by changing the electrical signal applied to the resonator.13. In a tunable laser system comprising a laser and a waveguide-coupledresonator, the waveguide-coupled resonator comprising first and secondwaveguides and a resonator having an adjustable refractive index inducedtunable resonance wavelength, a method for tuning the wavelength of thetunable laser system comprising: adjusting the refractive index of theresonator by applying an electrical signal to the resonator such thatthe resonance wavelength of the resonance corresponds to a selectedwavelength; coupling light emissions from the laser into the firstwaveguide; coupling the light emissions from the first waveguide intothe resonator through evanescent coupling, whereby substantially onlythe light emissions having a wavelength at the resonance wavelength ofthe resonator are coupled into the resonator; coupling light at theresonance frequency of the resonator from the resonator into the secondwaveguide; reflecting the light coupled into the second waveguide backtoward the resonator; coupling the reflected light from the secondwaveguide back into the resonator; coupling light at the resonancefrequency of the resonator from the resonator back into the firstwaveguide; and coupling light at the resonance frequency of theresonator from the first waveguide back into the laser.
 14. The methodof claim 13, wherein the electrical signal comprises a voltage.
 15. Themethod of claim 13, wherein the electrical signal comprises a current.16. The method of claim 13, wherein the light in the second waveguide isreflected using a mirror.
 17. The method of claim 13, wherein the lightin the second waveguide is reflected using a reflective coating on thesecond waveguide.
 18. The method of claim 13, wherein the lasercomprises a laser diode.
 19. The method of claim 18, further comprisingapplying a current to the laser diode below a lasing threshold of thelaser diode to generate the light emissions.
 20. The method of claim 19,further comprising applying enough current to the laser diode so thatlight at the resonance frequency of the resonator has enough gain toovercome optical losses associated with the system.
 21. The method ofclaim 20, wherein the laser diode has opposing first and second endfacets, the light emissions are emitted from the second end facet of thelaser diode, light at the resonance frequency of the resonator iscoupled from the first waveguide into the laser diode through the secondend facet, and the method further comprises increasing the currentapplied to the laser diode until light at the resonance frequency of theresonator becomes lasing and is outputted from the first end facet ofthe laser diode.
 22. The method of claim 13, further comprisingadjusting the resonance frequency of the resonator to another selectedfrequency by changing the electrical signal applied to the resonator.